Filling
<p>
Write a number in every blank square of the grid. When the grid is
full, every orthogonally connected group of identical numbers should
have an area equal to that number: so 1s always appear alone, 2s in
pairs, and so on.
<p>
To place a number, click the mouse in a blank square to select it,
then type the number you want on the keyboard. You can also drag to
select multiple squares, and then type a number to place it in all
of them. To erase numbers, select one or more squares in the same
way and then press Backspace.
